Rotational casting



July 17, 1962 v. E. VALENTINE RoTATIoNAL CASTING 3 Sheets-Sheet 1 Original Filed Sept. 5, 1957 INVENTOR VAUGHN E. VALENTINE BY 0% u .if mm ATTORNEYS July 17, 1962 v` E. VALENTINE' RoTATloNAL CASTING 3 Sheets-Sheet 2 Original Filed Sept. 5, 195'? CHARGING 8 CLOSING MOLDS NO ROTATION TEMP |50 F DISTR BuTl-oN PARTIAL GELLING (THIN LAYER,

l5-20 SEC.

INVENTOIL VAUGHN E. VALENTINE ATTORNEYS July 17, 1962 v. E. VALENTINE ROTATIONAL CASTING 3 Sheets-Sheet 3 Original Filed Sept. 5, 1957 SET FULLY GELLED COOLING 2-30 RPM UNTIL TEMP( |30 F INVENTOR. VAUGHN E VALENTINE ATTORNEYS United States The present invention relates to improvements in the rotational casting of synthetic resins such as vinyl plastisols and similar materials. More particularly, the invention relates to improved rotational casting methods.

In the rotational casting of intricate articles, e.g. the arms or legs of a play doll, it has been found that conventional techniques are not completely satisfactory. During the early stages of this art, much effort was expended in perfecting apparatus and methods for the casting of even uncomplex shapes such as the play balls or footballs. As this phase was perfected, increasing attention was paid to the problems of casting more intricate shapes. Apparatus then in use was modified in an effort to achieve the desired versatility. However, these efforts have met with only limited success.

Therefore, it is an object of the present invention to provide an improved method for the rotational casting of intricate articles having superior properties and ap-v pearance; such invention also being applicable to iniproved production of relatively uncomplex shaped articles.

Some further objects are: to provide an improved method which involves rotating mold carriers on their major and minor axes at different speeds and in selective sequence; to provide a more efficient method for simultaneous loading, curing and cooling of a series of molds; and to provide improved casting methods which are performed during the curing and cooling stages to produce an article having superior physical properties and appearance.

These and still other objects of the invention will be apparent in view of the following detailed disclosure including the drawings.

In the drawings:

FIG. l is a plan View showing schematically operation of the casting apparatus practicing the method of the invention;

FIG. 2 is a section taken substantially as indicated on line 2 2 of FIG. l;

FIG. 3 is a schematic representation partly broken away of an individual mold as shown in FIG. 2 depicting the mold charged and closed;

FIG. 3a is an enlarged fragmentary area of FIG. 3;

FIG. 3b is a schematic representation of means for biaxial rotation of the mold shown in FIG. 3;

FIG. 4 is a figure similar to FIG. 3 showing the initial distribution of the plastisol charged within the mold;

FIG. 4a is an enlarged fragmentary area of FIG. 4;

FIG. 4b is a schematic representation of the mold being rotated about one axis;

FIG. 5 is a figure similar to FIG. 3 showing the partial gelation of the plastisol charge within the mold;

FIG. Sais an enlarged fragmentary area of FIG. 5;

FIG. 5b is a schematic representation of the high speed rotation of the mold about one axis;

FIG. 6 is a gure similar to FIG. 3 showing the plastisol charged within the mold fully gelled;

FIG. 6a is an enlarged fragmentary area of FIG. 6;

FIG. 6 is a ligure similar to FIG. 6 showing the plastisol charge within the mold fused, or fully set;

FIG. 6a is an enlarged fragmentary area of FIG. 6';

FIG. 6b is a schematic representation of the rotation of the mold about dual axes;

atent ice FIG. 7 is a ligure similar to FIG. 3 depicting the plastisol charge within the mold as being completely fused and cooled;

FIG. 7a is an enlarged fragmentary area of FIG. 7;

FIG. 7b isa schematic representation of the mold being rotated about one axis during the cooling stage.

A preferred form of casting apparatus used to practice the method of the invention is shown in the drawing and indicated generally by the numeral 20. A detailed description of the preferred apparatus is disclosed in U.S. Patent No. 2,957,202, issued on the parent application filed on September 5, 1957, Iin the name of Paul Rekettye Vand Vaughn E. Valentine, Serial No. 682,099. The

. mold carriers are indicated generally by the numeral 22.

The curing oven is indicated generally by the numeral 24.

In the form of the machine 20' as shown, there are provided three equally spaced (120 apart) mold-carrying arms, indicated generally by the numeral 35. Each arm 35 includes a solid shaft 36 coaxially enclosed Within or by a tubular shaft 37. The shaft 36 isdriven to rotate the mold carriers 22 on their major axis. The shaft 37 is driven to rotate the mold carriers on their minor axis.

Selective rotation of shafts 36 and 37 may be controlled automatically by a clutch assembly (not shown), and the mold carrying arms 35 may be successively rotated through the stripping and loading (I), the curing (Il) and the cool-ing (III) stations manually or by an indexing mechanism.

After the molds 100 have been charged with a predetermined amount of liquid plastisol (FIG. 3), the outer section 97 is fitted on the shaft 43, a cap 102 is aligned with each mold 100, and the locking assembly 99 is tightened.

As used herein, the terms plastisol charge or resinous plastisol are intended to mean a mixture of synthetic resin(s) and plasticizers which: can be molded, cast or converted to continuous lms by the application of heat. The term synthetic resin(s) is intended to mean a thermoplastic, complex, substantially amorphous organic semi-solid or solid material (usually a mixture) built up by chemical reaction of comparatively simple compounds, approximating the natural resins in luster, fracture, comparative brittleness, insolubility in water, fusibility or plasticity, and some degree of rubber-like extensibility; but commonly deviating widely from natural resins in chemical constitution and behavior with reagents. More specifically, the terms include the vinyl polymers and copolymers such as polyvinyl chloride (P.V.C.).

Without in any sense limiting the scope of the invention to the proportions set forth, the following is a detailed description of one suitable formula for use in the manufacture of articles according to the invention, the compound formulation being known as a vinyl plastisol.

To l0() parts of a conventional polyvinyl chloride polymer, such as is well known to those skilled in the art, for example, the compound Geon 121, obtainable from the B. F. Goodrich Chemical Co., is added: parts of a conventional plasticizer, such as di(2ethylhexyl) phthalate (DOP), also Well known to those skilled in the art; 2 to 6 parts of a liquid barium-cadmium stabilizer; such as Advastab X23-30 obtainable from the- Advance Solvents & Chemicals Corp., `also well known to those skilled in the art, and inert pigment as required depending upon the color desired for the plastisol.

schematically depict the well-known increase of colloidal substance as the resin and plasticizer solvates during distribution, gelation and fusion. A more complete discussion of the gelling audfusion of plastisols is given in U.S. Patent No. 2,629,134 to Molitor.

At the loading and stripping stationV (I), shafts 36 and 37 are stationary `and the molds 1.60 are cooled to substantially room temperature, in 4any event less than 130 F. After the molds are charged, with a predetermined amount of plastisol, and closed, the casting machine turret is rotated toward the curing oven 24.

The temperature of the oven 24 is so chosen as toprcvide sufficient heat energy to bring the Walls of the cool molds upto the fusing temperature within the time allotted for the curing cycle. The exact temperature at which the oven is maintained will depend upon the oven insulation, the thickness and heat transfer rate of the mold walls, the size of the oven Vdoors 165, and other factors. Selection of the proper oven temperature is within the knowledge of those skilled in this art.

As the arm 35 enters the oven, shaft 37 is rotating at a rate selected from the range 2-30 r.p.m. Such rotation on the minor axis is of brief duration (2 to 5 seconds) and causes full distribution of the plastisol in theremote and intricate areas of the molds 100 (see FIGS. 4, 4c: land 4b). Of course, to accomplish this in the embodiment shown, the minor axis must be parallel to the ground. Such rot-ation also ensures that the cool molds will be heated at la uniform rate with no localized areas having signiiioant temperature differences.

Substantially simultaneously with the indexing of arm 35 at curing station (Il), and with the minor axis maintained parallel to the ground, shaft 37 isspun at a rateV selected from the range 50-150 rpm.V (FIGS. S-Sb). Such high speed rotation on the minor axis .is of longer duration (l5 to 2O seconds) and continues until la very thin gelled laye-r of pl'astisol isformed on the walls of the mold. It has been found that the centrifuga-l force generated by the high speed rotation, while heating the mold so that the outermost layer of plastisol is gelled, will be sufficient to cause the plastisol to remain in the remote l'and intricateareas of the mold during the nal curing and solidifying stages of the process.

Before the plastisol has gelled beyond the thin outer layer, Athe highspeed rotation is stopped, and simultaneous rotation on both axes at a rate selected from the range 2-30 rpm. takes place for a period of yabout seven minutes, or until the plastisol has fully gelled (FIG. 6) and set (FIG. 6'). After the plastisol charge has fused, the arm 3S which has been in the furnace, is indexed to the cooling station.

At the cooling station (III), the minor axis shaft 37 only is rotated (see FIG. 7) and the mold carriers 22 are iirst immersed in a water bath IGS. The temperature of the bath 198 is approximately 150 -F. and the molds are rotated therein for approximately 30 seconds. The temperature of the bath and the duration of rot-ation therein aire so chosen as to ensure that the solidified plastisol is quickly lowered to a temperature significantly below the setting temperature, so that there can be no deterioration of the product. After immersion cooling in the bath, the mold carriers Iare further cooled by slow rotation in air for the balance of the cycle (7 minutes or less). At the end ofthe cooling period, the temperature of the products within ythe mold should be 12d-130 F.

Specific operating times `and temperature for a speciiic vinyl plastisol formulation, have been described. It will be apparent however, that minor changes could be made therein without departing from the basic conceptsy of the invent-ion. Accordingly, the scope of the invention should be determined byv the fairly implied scope of the subjoined claims.

What is claimed is:

1. Ina process for casting intricate hollow articles from a vinyl resin plastisol, said process requiring a heating zone at the cur-ing temperature of said plastisol, the steps of, ylocating a series of molds radially of a first axis and transversely to a second axis, charging said molds when at a temperature below F. with a predetermined amount of plastisol, closing said molds, introducing said closed molds into said heating zone while rotating the molds at from 2-30 rpm. on said iirst axis, rotating said molds at from 50-15() r.p.rn. on said 'first axis as the temperature of the molds rises to the gelling temperature of said plastisol, and, before the plastisol has gelled beyond a thin outer layer, simultaneously rotating the molds at rates of from -2-30 rpm. on both axes as the tempera- .ture of the fmolds rises to the fusing temperature of the plastisol.

2. In a process according to claim 1, the additional steps of rapidly lowering the temperature of said molds to below the fusing temperature of said plastisol while rotating said molds at from 2-30 r.p.m. on said first axis, gradually lowering the temperature of said molds to a temperature below 130 F., and opening said cooled molds and removing the articles.

3. In a process for casting intricate hollow articles from a synthetic resin plastisol, said process requiring a heat-ing zone at the curing temperature of said plastiso-l, lthe steps of, locating a series of molds radially of a first axis and transversely to a second axis, charging said molds when outside said heating zone with a predetermnied amount' of plastisol, closing said molds, introducing said closed molds into said heating zone while rotating the molds at a slow rate on said rst axis, rotating the molds at a high rfate on said first axis as 4the temperature of the molds rises to the gelling temperature of said plastisol, and, before the plastisol has gelled beyond a thin outer flayer, simultaneously rotating the molds at a slow rate on both axes as the temperature of -the molds rises to the fusing temperature of said plastisol.

4.V In a process according to claim 3, the additionm steps of rapidly lowering the temperature of said molds to -below the fusing temperature of said plastisol while rotating said molds at a slow rate on said first axis, gradually lowering the temperature of said molds to a temperature below 130 F., and opening said cooled molds and removing the articles.

5. In a process for casting intricate hollow Karticles from synthetic resin plastisols, the steps of charging a cool mold with a predetermined amount of plastisol, closing the mold, distributing the plastisol in those portions of the mold forming the intricate portions of the article, subjecting said closed mold to strong centrifugal forces of rotation on a iirst axis while raising the temperature of the mold to the gellingrternperature of said plastisol, and, before the plastisol has gelled beyond a thin outer layer, further subjecting said mold to simultaneously applied weaker rotative forces on said first axis and on a second axis divergent therefrom while raising the temperature of the mold to the fusing temperature of said plastisol.

6. In a process according to claim 5, the additional steps of rapidly lowering the temperature of said molds to below the fusing'temperature of said plast-isol while `rotating said molds at a slow rate on said first axis, gradually lowering the temperature of said molds to a temperature below 130 F., and, opening said cooled molds and removingl the articles.

7. In a process for casting intricate hollow articles from synthetic resin plastisols, the'steps of charging a moid with a predetermined `amount of plastisol, closing the mold, distributing the plastisol in those portions of the mold form-ing the intricate portions of the article, subjecting said closed mold to a strong centrifugal force on a single axis lwhile raising the temperature of the mold to the gelling temperature of said plastisol, and before the plastisol has gel-led beyond a thin outer layer, further subjecting said mold to simultaneously applied weaker rotative forces having divergent axes while raising the temperature of the mold to the fusing temperature of said plastisol.

`8. For rotational casting of synthetic resin pllastisols, a process including the steps of, exposing a closed mold charged with a predetermined amount of plastisol to heat sufcient to gel the layer of pltast-isol in contact with the interior of said mold while the mold is subjected tto slow rotation and then strong centrifugal forces on a single axis, further heating said mold to a temperature sucient to fuse said plastisol while said mold is subjected to a plurality of rotative forces having divergent axes, yand rapidly lowering the temperature of said mold -to below the fusing temperature `of said mold while the mold is subjected to weaker centrifugal forces on said single axis.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Rotational `Casting of Plastisols, from Modern Plastics, September 1954, 4pages. Y 

1. IN A PROCESS FOR CASTING INTRICATE HOLLOW ARTICLES FROM A VINYL RESIN PLASTISOL, SAID PROCESS REQUIRING A HEATING ZONE AT THE CURING TEMPERATURE OF SAID PLASTISOL, THE STEPS OF; LOCATING A SERIES OF MOLDS RADIALLY OF A FIRST AXIS AND TRASVERSELY TO A SECOND AXIS, FHARGING SAID MOLDS WHEN AT 